A technology has been proposed in which a plurality of bits is transmitted in one symbol time in order to achieve high-speed and high-capacity optical transmission. In multivalue modulation, a plurality of bits is transmitted using one symbol. For example, quadrature phase shift keying (QPSK) and m-quadrature amplitude modulation (m-QAM, and “m=16, 64, 256, or the like” is satisfied) have been put into practical use. In addition, in polarization multiplexing, a signal is transmitted using two polarization beams orthogonal to each other.
Recently, the multivalue modulation is achieved by digital signal processing. For example, a transmitter includes a digital signal processing circuit and an in-phase/quadrature (I/Q) modulator. The digital signal processing circuit generates a drive signal from transmission data. The I/Q modulator generates an optical modulation signal by modulating a carrier light using the drive signal applied from the digital signal processing circuit. In addition, a receiver includes a front-end circuit and a digital signal processing circuit. The front-end circuit converts a received optical modulation signal into an electric field information signal. The digital signal processing circuit regenerates transmission data, based on the electric field information signal.
In the multivalue modulation in which the number of bits per one symbol is large, it is expected that the demand for perfection of the transmitter becomes severe. The imperfection of the transmitter causes waveform distortion of an optical modulation signal. As factors that cause the waveform distortion of the optical modulation signal, for example, there are the following examples.
(1) Nonlinearity of a driver or an I/Q modulator, or harmonic distortion
(2) Skew of a drive signal of an I/Q modulator
(3) Lack of an analog bandwidth of a digital-to-analog converter (DAC), a driver, an I/Q modulator, or wiring (substrate, cable, connector, or the like)
(4) Extinction ratio of an I/Q modulator
(5) Bias of each arm of an I/Q modulator
(6) Bias of π/2 phase shifter of an I/Q modulator
(7) Polarization beam extinction ratio of a polarization beam combiner
(8) Phase noise of a light source
(9) Deviation of the oscillation frequency of a light source (for example, for ITU-T grid)
In order to improve the quality of an optical modulation signal, it is desirable that waveform distortion caused by the above-described factor is detected, and compensated or suppressed. Japanese Laid-Open Patent Publication No. 9-64780 and Japanese Laid-Open Patent Publication No. 2011-135492 describe related technologies, for example.
The waveform distortion of the optical modulation signal caused by the imperfection of the transmitter (that is, the waveform distortion generated in the transmitter) may be detected, for example, using a digital coherent receiver. However, when a digital coherent receiver specific to detection of the waveform distortion is provided, cost of an optical transmission system is increased. The waveform distortion of the optical modulation signal may be detected using a digital coherent receiver in the opposite device of the optical transmission system. However, in this configuration, the detected waveform distortion includes distortion of a transmission path (transmission line) in addition to the distortion caused by the imperfection of the transmitter. Therefore, it is difficult to accurately detect the distortion caused by the imperfection of the transmitter, and it is difficult to accurately compensate the distortion.